The invention relates to gauges for indicating fluid pressure, and in particular to Bourdon tube type pressure gauges for use in connection with continuous monitoring of the pressure within fire extinguishers and the like.
Bourdon tubes are commonly used as the principal pressure sensitive unit in a variety of pressure gauges commercially available. The Bourdon tube has an angularly displaceable free end and pointer, the extent of displacement being proportional to the pressure applied. A calibrated dial plate is normally positioned on the casing beneath the pointer to translate tube displacement into values of pressure. Readout accuracy has been assisted heretofore by various calibration techniques limited to locating the pointer at only one position of the dial face.
However, it has not been possible heretofore to calibrate a Bourdon tube type gauge both by locating the pointer at the zero position and by aligning the dial indicia with the deflected position of the pointer resulting from the application of a pressure of selected value prior to completion of the gauge assembly. Accordingly, assembled gauges subjected to readout accuracy testing after assembly were often unable to pass quality control because of the normal variations in tolerance levels and consequent inconsistent "on-pressure" readings for the Bourdon tubes being utilized in the prior type of gauge construction. The Bourdon tube deflection, for various reasons does not always predictably correlate to fixed dial plate indicia of the gauge and many prior gauges have therefore been too often inaccurate. This has caused an unacceptably high post-assembly gauge rejection rate in the industry and contributes to increased costs and lower accuracy of the gauges.
Moreover, prior pressure tight gauges of the type described herein have often presented a hazard where the pressure responsive element or other pressure sensitive aspects of the gauges develop leaks or rupture to permit a pressure build-up within the gauge casing. Injury has occurred where the viewing crystal or lens blows out or fractures as a result of excessive pressure within the casing.
Furthermore, prior pressure gauges of the present type have required a plurality of pressure seals and other components to preserve the pressure tight assembly and to ensure adequate support for the pressure responsive element.